Derivatives of dipyrido[3,2-a:2′,3′-c]phenazine and its ruthenium complexes, influence of arylic substitution on photophysical properties

Article abstract of DOI:10.1039/b512773d

The synthesis and photophysical properties of a series of substituted dipyridophenazine (dppz) ruthenium complexes of the type [(tbbpy) ₂Ru(dppz-R₂)]²⁺ (where tbbpy = 4,4-tert-butyl-2,2-bipyridine and dppz = dipyrido[3,2-a

Full text of DOI:10.1039/b512773d

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Derivatives of dipyrido[3,2-a:2ꢀ,3ꢀ-c]phenazine and its ruthenium complexes,
influence of arylic substitution on photophysical properties†
Bernhard Scha¨fer,a Helmar Go¨rls,a Martin Presselt,b Michael Schmitt,b Ju¨rgen Popp,b William Henry,c
Johannes G. Vosc and Sven Rau*a
Received 8th September 2005, Accepted 18th January 2006
First published as an Advance Article on the web 14th February 2006
DOI: 10.1039/b512773d
The synthesis and photophysical properties of a series of substituted dipyridophenazine (dppz)
ruthenium complexes of the type [(tbbpy)2Ru(dppz-R2)]2+ (where tbbpy = 4,4-tert-butyl-2,2-bipyridine
and dppz = dipyrido[3,2-a:2ꢀ,3ꢀ-c]phenazine and R represents substitution at the 11 and 12 position
with: Br, phenyl, 4-tert-butyl-phenyl and para-biphenyl) are described. The ligands could be obtained in
high yields using Suzuki-type coupling reactions, an approach which also has been successfully applied
to the analogous dppz-Br2 ruthenium complex. All compounds are fully characterised by NMR, MS
and UV-vis spectroscopy. The solid state structures of dppz-bi-para-biphenyl and the ruthenium
complex [(tbbpy)2Ru(dppz-Br2)]2+ are also reported. The investigation of the free ligands reveals a
pronounced effect of the arylic substitution on absorption and emission properties. These properties are
mirrored in the corresponding complexes, which possess emission lifetimes of up to 900 ns. The
resonance Raman investigation of the complex [(tbbpy)2Ru(dppz-Br2)]2+ supports the assumption that
the excited state properties of the substituted complexes are related to the parent [(bpy)2Ru(dppz)]2+
compound, but that important differences may be expected based on the differences observed in the
lowest energy absorption band.
pyrazine moiety where the 3MLCT is finally localised. It is gener-
ally accepted that it is this state that is responsible for the unique
Introduction
Ruthenium complexes with dipyrido[3,2-a:2ꢀ,3ꢀ-c]-phenazine
(dppz) are intensively investigated due to their potential as
luminescent DNA sensors1–11 or as reversible electron carriers.12
Both functions strongly depend on the role of the phenazine part of
the dppz ligand as electron acceptor. In addition to this electronic
role structural features of the ligand are equally important. For
instance the planar geometry of the dppz ligand allows efficient
intercalation with helical DNA which is observed with high
equilibrium binding constants for Ru–DNA of up to 106 M−1.5 In
view of these potential applications substantial efforts have been
directed towards the design of new dppz-type ligand structures.13–16
To rationally design such new ligands it is helpful to consider the
course of events following photoexcitation with visible light. For
ruthenium dppz complexes an electron, formally originating from
a d orbital on the ruthenium centre, is transferred to a p* orbital
of the dppz ligand and this results in the formation of a long lived
3MLCT state.10,17–20 A currently discussed detailed mechanism for
photophysical properties of these complexes.21–23 The aim of this
contribution is the preparation of novel ligands that will lead to
metal complexes with significantly improved excited state proper-
ties, such as longer excited state lifetimes. This may be achieved by
delocalisation of the charge in the final triplet state. One way of
achieving this is by the introduction of phenyl-type substituents.
In this contribution we present a series of substituted dppz lig-
ands and their corresponding ruthenium polypyridyl complexes,
see Fig. 1. The reasons for their selection is outlined above but
we also wish to emphasise that the development of an efficient
synthetic method for introducing substituents in the dppz frame
using organometallic reactions is also a major point in this study.
3
the population of this MLCT involves initial population of the
1MLCT localised on the bipyridine part of the dppz ligand with
subsequent intraligand electron transfer to the phenazine based
aInstitut fu¨r Anorganische und Analytische Chemie, Friedrich-Schiller-
Universita¨t, Lessingstr. 8, 07743, Jena, Germany. E-mail: sven.rau@uni-
jena.de; Fax: +49 3641 948102; Tel: +49 3641 948113
bInstitut fu¨r Physikalische Chemie, Helmholzweg 4, Friedrich-Schiller-
Universita¨t, 07743, Jena, Germany
cNational Centre for Sensor Research, School of Chemical Sciences, Dublin
City University, Dublin 9, Ireland
† Electronic supplementary information (ESI) available: Detailed NMR
spectra of the ligands LR and complexes RuLR. See DOI: 10.1039/
b512773d
Fig. 1 Naming of ruthenium complexes and corresponding ligands.
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To avoid solubility problems we utilised tert-butyl substituted
complexes since this ensures high solubility in organic solvents.
Since it has recently been emphasised that the substitution of the
ancillary ligands may have an influence on the entantioselectivity
of the interaction of the complexes with DNA,24,25 we were also
interested in obtaining more detailed structural information on
substituted dppz–ruthenium complexes.
The novel ligands L1 to L4 have been prepared to illustrate the
potential of the building block approach utilising organometallic
C–C coupling reactions.
L3 1H NMR (ppm; d6-DMSO) 7.43 H7, H8, H9 (10H, m); 7.70
H6, H5 (8H, m); 7.91 H2(2H, dd); 8.31 H4(2H, s); 9.19 H3(2H,
d(lc)); 9.44 H1(2H, d(lc)). MS (DEI, EI+ Q1MS) m/z = 586
(100%) (M+).
1
L4 H NMR (ppm, CD2Cl2) 1.321 CH3 (tert-butyl, 18H, s);
7.325 H6, H5(8H, m); 7.794 H3 (2H, dd); 8.361 H4 (2H, s); 9.215
H3 (2H, d(lc)); 9.622 H1 (2H, d(lc)); MS (DEI, EI+ Q1MS) m/z =
546 (30%) (M+), m/z = 531 (35%) (M+ − CH3).
Complexation of L1–L4
Ru(tbbpy)2Cl2 (0.1 g, 0.14 mmol) and the corresponding ligand
(0.14 mmol) were suspended in DMF/H2O (80 ml/10 ml) and
heated at reflux for 3 h using microwave irradiation (150 W).
The solvent was removed under reduced pressure. The complexes
were recrystallized from an ethanol/aqueous NH4PF6 mixture and
further purified using chromatography using a gradient changing
from EtOH to EtOH/H2O/KNO3. Yield 80–85%. For a detailed
assignment of the chemical shifts to the corresponding atom see
the electronic supplementary information (ESI).†
RuL1 (ppm CD3CN) 1.347 (18H, s); 1.441 (18H, s); 7.233 (2H,
d); 7.466 (2H, d); 7.565 (2H, d); 7.659 (2H, d); 7.888 (2H, dd);
8.143 (2H,d); 8.476 (2H, s); 8.515 (2H, s); 8.844 (2H, s); 9.556
(2H, d) MS (ESI, MeOH,): m/z = 1223.2 ([M − (PF6)]+, 100%).
RuL2 (ppm CD3CN) 1.327 (18H, s); 1.434 (18H, s); 7.256 (2H,
d(lc)); 7.353 (10H, m); 7.475 (2H, d(lc)); 7.620 (2H, d); 7.694 (2H,
d); 7.896 (2H, dd); 8.146 (2H, d(lc)); 8.422 (2H, s); 8.512 (2H, s);
8.551 (2H, s); 9.589 (2H, d) MS (ESI, MeOH,): m/z = 1217.5
([M − (PF6)]+, 100%); m/z = 536.4 ([M − 2(PF6)]2+).
Experimental
Unless otherwise noted, all Pd catalyzed cross coupling reactions
were conducted under an atmosphere of dry, deoxygenated argon
using standard Schlenk techniques. Ru(tbbpy)2Cl2,26 phenO2
5
and 4,5-dibromo-orthophenylendiamine27 were prepared using
literature methods. Acetonitrile was distilled from CaH2. Toluene
was distilled from sodium benzophenone ketyl under an argon
atmosphere prior to use. All other solvents were used as received.
Infrared spectra were recorded using a Perkin-Elmer 2000 FT-
1
IR; H NMR spectra were recorded on a Bruker 400 MHz/200
Mhz spectrophotometer. The mass spectra were obtained using
a SSQ 170, Finnigan MAT at the Friedrich Schiller University
Jena. Electrospray-Mass spectra were recorded on a Finnigan
MAT, MAT 95 XL. The positive ES mass spectra were obtained
with voltages of 3–4 kV applied to the electrospray needle. The
microwave assisted reactions were carried out using the Microwave
Laboratory Systems MLS EM-2 microwave system.
RuL3 (ppm CD3CN) 1.277 (18H, s); 1.471 (18H, s); 7.412 (10H,
m); 7.211 (4H, d); 7.272 (4H, d); 7.348 (2H, d); 7.522 (2H, d); 7.772
(4H, d); 7.799 (2H, dd); 7.968 (2H, s); 8.241 (2H, d); 8.571 (2H,
s); 8.639 (2H, s); 9.175 (2H, d); MS (ESI, MeOH): m/z = 1369.2
([M − (PF6)]+, 69%); m/z = 611.9 ([M − 2(PF6)]2+, 100%).
RuL4 (ppm CD3CN) 1.312 (18H, s); 1.329 (18H, s); 1.468 (18H,
s); 7.228 (8H, m); 7.328 (2H, d(lc)); 7.505 (2H, d(lc)); 7.703 (2H,
d); 7.736 (2H, d); 7.774 (2H, dd); 8.031 (2H, s); 8.161 (2H, (lc));
8.517 (2H, s); 8.566 (2H, s); 9.268 (2H, d(lc)) MS (ESI, MeOH):
m/z = 1329.0 ([M − PF6]+, 100%), m/z = 531 ([M − 2PF6]2+,
20%).
Synthesis of Br2dppz (L1)
Phendione (2 g, 9.5 mmol) and 4,5-dibromo-orthophenylendia-
mine (2.534 g, 9.5 mmol) were refluxed in absolute EtOH (300 ml)
for 8 h. The volume of the solvent was reduced to 50 ml and cooled
in a refrigerator over night. dppzBr2 was collected by filtration.
1
Yield 90%. H NMR (ppm; CDCl3) 7.730 phen(2H, dd); 8.559
H4(2H, s); 9.233 phen(2H, d(lc)); 9.529 phen(2H, d(lc));EI-MS
m/z = 440 (100%) matching isotopic pattern.
Typical procedure for preparation of L2 from uncoordinated
Br2dppz (L1)
Suzuki-reaction at the complex (RuL1)
A Schlenk vessel was charged with dppzBr2 (0.250 g, 0.568 mmol)
and the corresponding boronic acid (1.136 mmol), Pd(PPh3)4
(0.013 g, 0.0113 mmol (1.0 mol %)) in 50 ml dry toluene and
15 ml of oxygen free solution of 2 M aqueous Na2CO3. The
suspension was refluxed under argon for three days. After cooling
to room temperature the reaction mixture was taken up in 100 ml
water followed by extraction with chloroform. The solvent of the
combined organic layers was removed and the crude product dried
under vacuum.
RuL1(PF6)2 (0.150 g, 0.11 mmol), phenyl boronic acid (0.028 mg,
0.23 mmol) and Pd(PPh3)4 (12 mg, 8.8 × 10−6 mol (4 mol%)) were
suspended in 50 ml acetonitrile and 15 ml of a oxygen free solution
of 2 M aqueous Na2CO3 and refluxed for 3 days. After cooling
to rt 100 ml of water were added followed by extraction with
dichloromethane. The organic layers were collected, the solvent
was removed under reduced pressure and the residue was cleaned
by column chromatography collecting the main band (using silica,
EtOH, EtOH/H2O, KNO3). Yield 67%. Analytical data obtained
are identical to those observed for the sample obtained from
complexation of L2 to the ruthenium precursor.
Column chromatography (using silica and a gradients eluent
system starting with CHCl3 and changing to CHCl3/MeOH (9 :
1). The fluorescent main band was collected and yields pure L2.
Yield 60–80%.
Crystal structure determination
1
L2 H NMR (ppm; CDCl3) 7.305 phenyl (10H, s); 7.808 H2-
phen (2H, dd); 8.363 H4 (2H, s); 9.300 H3-phen (2H, d); 9.626
H1-phen (2H, d); MS (DEI, EI+ Q1MS) m/z = 434 (100%) (M+).
The intensity data for the compounds were collected on a Non-
ius Kappa CCD diffractometer, using graphite-monochromated
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Mo-Ka radiation. Data were corrected for Lorentz and polariza-
tion effects, but not for absorption effects.28,29
photon counting apparatus (TCSPC) comprised of two model
J-yA monochromators (emission and excitation), a single pho-
ton photomultiplier detection system model 5300, and a F900
nanosecond flashlamp (N2 filled at 1.1 atm pressure, 40 kHz or
0.3 atm pressure, 20 kHz) interfaced with a personal computer via
a Norland MCA card. A 410 nm cut off filter was used in emission
to attenuate scatter of the excitation light (337 nm); luminescence
was monitored at the kmax of the emission. Data correlation
and manipulation was carried out using EAI F900 software
version 6.24. Samples were deaerated for 30 min using argon
prior to measurements followed by repeated purging to ensure
complete oxygen exclusion. Emission lifetimes were calculated
using a single-exponential fitting function, Levenberg–Marquardt
algorithm with iterative deconvolution (Edinburgh instruments
F900 software). The reduced v2 and residual plots were used to
judge the quality of the fits. Lifetimes are 5%.
The structures were solved by direct methods (SHELXS30)
and refined by full-matrix least squares techniques against Fo2
(SHELXL-97).31 The hydrogen atoms (without the disordered
toluene molecules of L3) were included at calculated positions with
fixed thermal parameters. All non-hydrogen atoms were refined
anisotropically.31 XP (SIEMENS Analytical X-ray Instruments,
Inc.) was used for structure representations. X-Ray suitable
crystals of RuL1 were grown from acetonitrile/hexane solutions.
Crystal data for RuL1. C54H56Br2F12N8P2Ru·2C2H3N, Mr =
1450.01 g mol−1, red-brown prism, size 0.03 × 0.03 × 0.02 mm3,
¯
triclinic, space group P1, a = 12.7435(3), b = 15.0747(3), c =
◦
˚
16.6350(4) A, a = 91.617(1), b = 97.359(1), c = 98.611(1) ,
V = 3129.9(1) A , T = −90 ◦C, Z = 2, qcalcd. = 1.539 g cm−3,
3
˚
l(Mo-Ka) = 16.6 cm−1, F(000) = 1464, 22044 reflections in
h(−16/14), k(−19/18), l(−21/21), measured in the range 3.91◦ ≤
H ≤ 27.49◦, completeness Hmax = 98.5%, 14161 independent
reflections, Rint = 0.031, 10601 reflections with Fo > 4r(Fo), 760
Resonance Raman spectroscopy
The Fourier transform off resonance FT-Raman spectrum of
the solid complex RuL1 was recorded with a Bruker IFS 120
HR spectrometer with an integrated FRA 160 Raman module
and liquid nitrogen cooled germanium detector. The 1064 nm
radiation of a Nd:YAG laser was employed for excitation. The
non-resonant Raman spectra of the solid complex [Ru(tbbpy)3]2+
excited at 830 nm was taken with a micro Raman setup (LabRam
invers, Jobin–Yvon–Horiba). The spectrometer has a focal length
of 800 mm and is equipped with 300 lines/mm grating. The applied
laser power was about 8 mW. The scattered light was detected by
a CCD camera operating at 220 K. An Olympus MLPlanFL 50
objective focused the laser light on the solid samples.
The resonance Raman spectra were recorded with a conven-
tional 90◦-scattering arrangement. The excitation lines at 514.5,
488 and 458 nm provided by an argon ion laser (Spectra Physics
Model 2085) served for resonant excitation in the range of the
MLCT absorption band (see Fig. 7 later). To avoid the heating
of the samples a rotating cell was utilized. No changes in the
absorption spectra of the complexes could be observed after
exposure to the resonant laser light. Moreover, the spectral lines
observed under resonant conditions were also present in the off-
resonant spectrum of the solid complexes, which was obtained with
excitation at 830 nm. These findings indicate that the scattering due
to possible photoproducts was minimal. The scattered light was
collected with a photo objective (f = 50 mm, 1 : 0.7) and analysed
with a Spex 1404 double monochromator. The dispersed Raman
stray light was detected with a Photometrics model RDS 200 CCD
Raman detector system. The concentration of the solutions was
optimized to obtain a maximum signal-to-noise ratio and was in
the millimolar range.
parameters, 12 restraints, R1obs = 0.067, wR2 = 0.169, R1all
=
obs
0.096, wR2all = 0.192, GOOF = 1.032, largest difference peak and
−3
˚
hole: 2.672/−1.039 e A .
Crystal data for L3. C42H26N4·C7H7, Mr = 677.79 g mol−1,
colourless prism, size 0.06 × 0.06 × 0.05 mm3, triclinic, space
¯
˚
group P1, a = 11.3762(6), b = 12.7563(7), c = 13.6865(5) A, a =
◦
3
˚
97.221(3), b = 102.933(3), c = 110.078(2) , V = 1773.1(1) A ,
T = −90 ◦C, Z = 2, qcalcd. = 1.270 g cm−3, l(Mo-Ka) = 0.75 cm−1,
F(000) = 710, 12620 reflections in h(−14/14), k(−15/16),
l(−17/17), measured in the range 2.08◦ ≤ H ≤ 27.46◦, complete-
ness Hmax = 98.8%, 8025 independent reflections, Rint = 0.033, 4464
reflections with Fo > 4r(Fo), 451 parameters, 0 restraints, R1obs
=
0.089, wR2 = 0.243, R1all = 0.152, wR2 = 0.293, GOOF =
obs
all
−3
˚
1.027, largest difference peak and hole: 0.779/−0.488 e A .
CCDC reference numbers 290998 and 290999.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b512773d
Electronic spectroscopy
UV-vis absorption spectra (accuracy 2 nm) were recorded on
a Varian Cary 50 Scan spectrometer interfaced with a Dell
optiplex GXI PC using Win UV Scan Application 2.00 and
on an Analytikjena Specord S 600 spectrometer with standard
software based tools. Emission spectra (accuracy 5 nm) were
recorded at 298 K using a Perkin-Elmer LS50B luminescence
spectrophotometer, which was equipped with a red sensitive
Hamamatsu R298 PMT detector and interfaced with an Elonex
PC466 employing Perkin-Elmer Fl WinLab custom built software.
Emission and excitation slit widths were 10 nm. Emission spectra
for the ligands L2 to L4 were recorded at only 1% transmission
due to large emission intensities. Emission spectra are uncorrected
for photomultiplier response. 10 or 2 mm path length quartz cells
were used for recording spectra.
Results and discussion
Synthesis
All complexes under investigation are based on 4,4ꢀ-di-tert-
butyl-2,2ꢀ-bipyridine (tbbpy) to increase the solubility of the
ruthenium complexes in less polar organic solvents. This is
helpful during the development of new synthetic methods which
rely on organometallic reactions.32,33 The new ligands L1 to
Emission lifetime measurements
Luminescence lifetime measurements were obtained using an
Edinburgh Analytical Instruments (EAI) time-correlated single-
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L4 rely on the accessibility of 4,5-diamino-1,2-dibromobenzene
which was conveniently prepared using literature methods27 and
coupled to phenanthrolin-5,6-dion resulting in 11,12-dibromo-
dipyrido[3,2-a:2ꢀ,3ꢀ-c]-phenazine (L1) in nearly quantitative yield.
Both bromine functions in L1 are easily substituted by aromatics
using conventional Suzuki-reactions utilising {Pd(P(Ph)3]4} as
catalyst yielding L2–L4 in good yields (see Fig. 2).
of the pyrazine ring and the plane of the C13–C18 aromatic ring
in the ligand of the neighbouring complex at (1 − x, 2 − y, 1 −
z) as can be seen in Fig. 4. This arrangement is very similar to
11,12-dicyano-dppz ruthenium complexes.34
˚
Fig. 4 Solid-state structure of RuL1 relevant bond length in A: Ru–N1
2.063(4), Ru–N2 2.065(4), Ru–N5 2.051(4), Ru–N6 2.052(4), Ru–N7
2.053(4), Ru–N8 2.056(4), Br1–C15 1.880(5), Br2–C16 1.894(5); and
angles in ◦: N1–Ru–N2 79.16(16), N5–Ru–N6 78.61(16), N7–Ru–N8
78.17(17); additional “A” letter indicates symmetry equivalent position
of the complex at (1 − x, 2 − y, 1 − z).
Fig. 2 Synthetic route to aryl substituted dppz ligands.
The successful transformation of the cationic ruthenium com-
plex RuL1 using the Suzuki-protocol into RuL2, with 70% yield,
illustrates the potential of organometallic coupling reactions (see
Fig. 5) to selectively modify precursor complexes.32,33 No signifi-
cant advantages were identified when comparing the organometal-
lic coupling reactions starting from L1 or RuL1. Suzuki coupling
of the complexed ligand opens however the route towards the
introduction of substituents representing potential coordination
sites, such as different pyridines, which would interfere with
complexation if introduced into the free ligand.
The nature of the products was unambiguously identified
using multidimensional NMR methods, MS and UV-vis/emission
spectroscopy. In addition the X-ray structure of L3, is shown in
Fig. 3. The dppz frame is nearly ideally planar (deviation from
˚
planarity 0.038 A) whereas the two biphenyl substituents are
twisted out of the dppz plane by 47◦ (C16, C17, C19, C24) and
39◦ (C17, C16, C31, C36).
Fig. 3 Solid-state structure of L3.
Complexation of L1–L4 to (tbbpy)2Ru2+ fragments [Ru] is
achieved using microwave irradiation.26 The full characterisation
of all complexes with multidimensional NMR methods, MS and
UV-vis/emission spectroscopy is outlined in the experimental part
and in Table 1. In addition the solid-state structure of RuL1 has
been determined and is shown in Fig. 4. In the solid state structure
of RuL1 the deviation from planarity within the Br2dppz ligand
Fig. 5 Complex based Suzuki-reactions.
Electronic properties
˚
is 0.038 A which is negligible. That there is a significant p–p
interaction between neighbouring Br2dppz moieties is apparent
All four ligands display absorption maxima around 400 nm. A
clear influence of the peripheral substitution pattern on the shape
˚
in the solid state with a distance of 3.54 A between the centroid
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of the long wavelength maxima is observed. For example, L1
features two distinct maxima at 393 nm and 374 nm (shoulder
at 381 nm). similar to unsubstituted dppz.13 The introduction
of aromatic substituents leads to a red shift of the absorption
maxima and loss of the band at around 370 nm. The bathochromic
shift is most pronounced for the bi-biphenyl substituted L3. This
ligand shows only a single broad absorption at 408 nm. All
three aromatically substituted ligands (L2 to L4) show relatively
strong emission signals between 433 (L2) nm and 470 (L3) nm in
dichloromethane, see Fig. 6, whereas L1 displays only very weak
emission at 420 nm. The largest stokes shift (>60 nm) is observed
for the biphenyl substituted L3.
Fig. 6 Absorption and emission spectra of L2 in dichloromethane
solution (c(L2) = 8.6 × 10−6 mol l−1).
The excited state lifetimes of L2–L4 have been determined.
They range from 0.6 to 2 ns, (see Table 1). The absorption and
emission data of L1 to L4 indicate that the substitution of the dppz
frame with aromatic moieties results in very significant changes in
the electronic properties of the ligands. It is important at this
stage to note that since the ligand based electronic transitions
of the LR ligands are at least partly in the visible range of the
spectrum and therefore in the same range as those expected for
ruthenium polypyridyl-type MLCT the lowest energy transition in
a corresponding complex may contain significant p–p* character.
All four complexes show absorption properties which are
common for this class of ruthenium polypyridyl compounds. Their
oxidation potential is not affected by the peripheral substitution
at the dppz ligand and is observed at 0.82 V vs Fc/Fc+. This
suggests that the introduction of the substituents has no significant
influence on the metal based ground state. The absorption spectra
in acetonitrile are depicted in Fig. 7. The absorption spectrum of
RuL1 is very similar to the related compound [Ru(bpy)2dppz]2+
showing a MLCT band at 444 nm and a dppz based p–p*
transition at shortly below 400 nm.15 Introduction of more
extensive aromatic substituents leads to a red shift of the p–p*
transition, which ultimately overlaps increasingly with the Ru-p*
MLCT band. This results in an increase of the extinction within
the MLCT region from 17 000 for RuL1 at 444 nm to 26 000 for
RuL3 at 414 nm.
The luminescence data for all four complexes could be obtained
in acetonitrile and dichloromethane. A profound influence of the
solvent on emission wavelength is evident by the ca. 20 nm red shift
of the emission wavelength upon going from dichloromethane to
acetonitrile. Similar solvent dependencies have been reported for
related ruthenium–dppz complexes.37,38 The most obvious impact
of substitution at the dppz core is observed for RuL1, which is
non-emissive even in CaH2 dried acetonitrile, whereas relatively
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Fig. 7 Absorption spectra of RuL1 to RuL4 in acetonitrile.
Fig. 8 Resonance Raman spectra of RuL1 dissolved in acetonitrile excited
at three different wavelength (514, 488, 458 nm) lying within the first
absorption band of RuL1(see Fig. 7). The resonance Raman band of
[Ru(tbbpy)3]2+ dissolved in dichloromethane excited at 458 nm in resonance
close to the maximum of the first absorption and is shown for comparison.
For details see text.
strong luminescence is observed in dichloromethane. A trend of
prolonged lifetimes for the aryl substituted complexes RuL2–RuL4
is evident with the longest lifetime of 929 ns observed for RuL4 in
dichloromethane. The investigation of the lifetimes of the excited
state of all complexes in both solvents reveals also a strong solvent
dependency. All lifetimes in acetonitrile are only about 25% of the
values obtained in dichloromethane. The lifetime of the excited
states of RuL1 in dichloromethane is comparable with literature
values obtained for the monobromo-dppz ruthenium complex.15
Detailed photophysical data are compiled in Table 1.
Resonance Raman of RuL1
Resonance Raman spectroscopy is an extremely capable tool to
characterise the molecular degrees of freedom involved in the
initial structural changes of photochemically active systems i.e. to
characterise the Franck–Condon-Point of an electronic transition.
The resonance condition arises when the wavelength used to excite
the Raman scattering lies within the electronic absorption band,
causing the vibrational modes involved in the electronic transition
to be selectively enhanced (by a factor of 106 compared to non-
resonant excitation). Resonance Raman spectroscopy is therefore
the method of choice for investigating the location of the initially
formed MLCT in heteroleptic complexes where two different
ligand sites are possible.
Fig. 8 shows the resonance Raman spectra of RuL1 excited at
three different wavelengths within the first MLCT transition. The
vibrational modes at 1599, 1573, 1330 and 1173 cm−1 grow in
intensity when the excitation wavelength is shifted from 514 nm at
the red edge of the absorption band to 458 nm near the maximum
of the lowest energy absorption feature and these modes can be
assigned to the L1-ligand. This is confirmed by the comparison
between the resonance Raman spectrum of RuL1 with that of
[Ru(tbbpy)3]2+. The main modes enhanced in the resonance Raman
spectrum of RuL1 are not observed in the spectrum of the latter
compound and must therefore be due to L1. This assignment is
further supported by looking at the off resonance Raman spectra
of both molecules plotted in Fig. 9. The fact that the modes
which are enhanced most belong to the L1-ligand leads us to
the conclusion that exciting RuL1 within the MLCT transition at
458 nm involves mainly the L1-ligand rather than tbppy.
Fig. 9 Off resonance Raman spectra of RuL1(kexc = 1064 nm) and
[Ru(tbbpy)3]2+ (kexc = 830 nm).
be due to tbbpy. The fact that possibly also tbbpy-modes may
be enhanced suggests that Ru-to-tbbpy-type MLCT transitions
may also contribute to the absorption envelop. This assignment
is based on literature values from Meyer et al.39 and McGarvey
et al.20 Theoretical calculations which are currently under way may
help to shed more light on this issue.40
Conclusion
In conclusion we have shown that it is possible to obtain ruthenium
polypyridyl-type complexes containing substituted dppz-type lig-
ands using Suzuki coupling reactions starting either from the
free ligand dibromo-dppz, L1, or from its ruthenium complex
RuL1. For the compounds reported in this contribution the
results obtained from both synthetic approaches are very similar.
However, the use of a metal complex rather than the free ligand
offers significant advances when the substituents to be introduced
either have coordinating properties or would not be stable at
Importantly, however, other modes seen in the resonance
Raman spectrum at 1615, 1541, 1376, 1320 and 1211 cm−1 might
2230 | Dalton Trans., 2006, 2225–2231
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¨
the high temperature conditions needed to prepare the metal
compounds.
10 C. G. Coates, J. Olofsson, M. Coletti, J. J. McGarvey, B. Onfelt, B.
Lincoln, B. Norde´n, E. Tuite, P. Matousek and A. W. Parker, J. Phys.
Chem. B, 2001, 105, 12653.
The second important observation is that the absorption spectra
of the free ligands are very dependent on the substitution pattern.
Importantly, with increasing aryl character the ligand centred p–
p* transition of the LR ligand moves to lower energy and overlaps
considerably with the MLCT transition. This effect has two
potentially important consequences; one relates to the application
of the ruthenium dppz complexes as luminescent sensors, in that
higher extinction coefficients in the visible region will result in a
better sensor (ruthenium complex) to analyte (for instance DNA)
ratio.
In addition this shift may affect the so called “dark state”
of the dppz molecule.35 The importance of intraligand electron
transfer, from the initially populated bpy 1MLCT to the phenazine
based 3MLCT for explaining the unusual light switch mechanism
observed for the ruthenium dppz complexes has recently been
emphasised by Batista and Martin.36 Expansion of the aromatic
frame as evident in L2 to L4 may be expected to influence the rate
constants for this process and therefore lead to new insights into
this phenomenon.
11 G. Pourtois, D. Beljonne, C. Moucheron, S. Schumm, A. Kirsch-De
Mesmaeker, R. Lazzaroni and J.-L. Bredas, J. Am. Chem. Soc., 2004,
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12 D. A. McGovern, A. Selmi, J. E. O’Brien, J. M. Kelly and C. Long,
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14 P. Aguirre, R. Lo´pez, D. Villagra, I. Azocar-Guzman, A. J. Pardey and
S. A. Moya, Appl. Organomet. Chem., 2003, 17, 36.
15 N. J. Lundin, P. J. Walsh, S. L. Howell, J. J. McGarvey, A. G. Blackman
and K. C. Gordon, Inorg. Chem., 2005, 44, 3551 and references cited
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16 M. K. Kuimova, W. Z. Alsindi, J. Dyer, D. C. Grills, O. S. Jina, P.
Matousek, A. W. Parker, P. Portius, X. Zhong Sun, M. Towrie, C.
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19 E. M. Kober and T. J. Meyer, Inorg. Chem., 1982, 21, 3967.
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Acknowledgements
25 P. P. Pellegrini and J. R. Aldrich-Wright, Dalton Trans., 2003, 176.
26 S. Rau, B. Scha¨fer, A. Gru¨ßing, S. Schebesta, K. Lamm, J. Vieth, H.
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Financial support by the DFG (SFB 436) is gratefully acknowl-
edged. S.R. acknowledges financial support by the DFG (RA
1017/1-1 and 1-2). W.H. would like to thank Enterprise Ireland
(Grant Number SC/2003/74) for financial support. We would
like to thank Professor D. Walther for helpful discussions and
encouragement.
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